overlay process window

**Overlay Process Window** defines the **range of overlay errors within which the device still functions correctly** — specified by overlay tolerance or budget, the process window is the maximum allowable registration error between layers before shorts, opens, or electrical failures occur. **Overlay Budget Components** - **Scanner Contribution**: Stage positioning accuracy, lens distortion, inter-field stitching — the lithography tool's overlay error. - **Process Contribution**: Wafer distortion from thermal processing, film stress, CMP — process-induced overlay errors. - **Metrology Contribution**: Measurement uncertainty — the error in measuring the overlay itself. - **Total Budget**: $OV_{total}^2 = OV_{scanner}^2 + OV_{process}^2 + OV_{metrology}^2$ — RSS (root sum square) combination. **Why It Matters** - **Yield Cliff**: Overlay errors beyond the process window cause catastrophic yield loss — edge placement errors create shorts or opens. - **Shrinking Budget**: <5nm nodes require <2nm total overlay — every component must improve. - **Design Rules**: Overlay budget determines minimum design rules for contacts-to-gates and via-to-metal connections. **Overlay Process Window** is **the alignment tolerance budget** — the total allowable registration error partitioned across tool, process, and metrology contributions.